1. Field of the Invention
The present invention relates to an acceleration sensor used for measuring acceleration or detecting vibration; and a method for producing the same. More particularly, the present invention relates to a high-performance acceleration sensor of a small size and a method for producing the same.
2. Description of the Related Art
A piezoelectric vibrator is used for a mechanical filter, an oscillator, an actuator or the like and it is made of a piezoelectric material such as a piezoelectric ceramic, quartz or lithium niobate. A suitable oscillation mode such as bending oscillation is selected according to a frequency range to be used. When an alternating electric voltage is applied to a piezoelectric vibrator, it is oscillated due to the piezoelectric effect with a low oscillation loss. Such a piezoelectric vibrator has large mechanical quality coefficients Q.sub.n and large coupling coefficients. Therefore, a filter can have a wider range and a vibrator has a better frequency precision than an LC filter and a vibrator using a capacitor and a coil.
In order to utilize such features of a piezoelectric vibrator, it is important to support or fix a piezoelectric vibrator correctly and surely at a node where the oscillation amplitude becomes zero so as not to suppress oscillations generated in the piezoelectric vibrator in a holding structure thereof.
A node in a piezoelectric vibrator is a point or a line with zero area. Therefore, it is desirable that the holding structure holds a piezoelectric vibrator at a point or at a line. However, it is impossible practically to hold a piezoelectric vibrator at a point or at a line. Further, it is also needed to hold it stably. Then, a piezoelectric vibrator is practically held at a point-like area or at a linear area by sacrificing the characteristics thereof somewhat.
Priorly, a piezoelectric vibrator is supported or fixed around a node with use of springs. However, this technique has following problems: (1) It is difficult to set positions for supporting or fixing a piezoelectric vibrator correctly around a node. (2) If a piezoelectric vibrator is supported or fixed strongly or a strong force is applied to the piezoelectric vibrator for supporting or fixing it surely, the characteristics of the piezoelectric vibrator are deteriorated or the piezoelectric vibrator is destroyed mechanically. (3) It is needed to support or fix a piezoelectric vibrator on a certain area in order to hold it at a particular posture. However, because a node of the. piezoelectric vibrator is a point or a line with no area, this means that vibrating portions in the piezovibrator are held or fixed. Therefore, vibrations are suppressed and characteristics of the piezoelectric vibrator are deteriorated.
FIG. 1 illustrates a concept of holding of a piezoelectric vibrator 1 at points. Driving electrodes 2 are formed on two opposing principal planes of a piezoelectric vibrator 1 of a square plate made of a piezoelectric material. When an alternating voltage is applied to the driving electrodes 2, expansion vibrations of the square plate are generated. Because the vibrations has a node point at a center of the square plate, the piezoelectric vibrator 1 is held by holders 3 to interpose it at two sides with an applied pressure. A top of the holder 3 has a flat region with a certain area, and the piezoelectric vibrator 1 is held stably by abutting it to the flat regions of the holder 3 at two principal planes. Further, the elastic property of the holder 3 buffers vibrations propagated from the outside.
FIG. 2 illustrates a concept of holding of a piezoelectric vibrator 1 at a line. Driving electrodes 5 are formed on two opposing principal planes of a piezoelectric vibrator 4 of a rectangular plate made of a piezoelectric material. When an alternating voltage is applied to the driving electrodes 5, longitudinal vibrations of the rectangular plate are generated having main vibrations along a longitudinal direction of the rectangular plate. Because the vibrations has a node line at a center line of the rectangular plate, the piezoelectric vibrator 4 is held at two principal planes with an applied pressure by interposing it between two holders 6. A top of the holder 6 has a flat region with a certain area, and the piezoelectric vibrator 4 is held stably by abutting the flat regions thereto at two sides. Further, the elastic property of the holder 6 buffers vibrations propagated from the outside.
As described above, prior art holding structures hold a piezoelectric vibrator directly with holders at point-like areas or at line-like areas having a finite area. Therefore, if the contact areas for holding are decreased, holding becomes unstable. Further, when a shock is given for example by falling it to the ground, the posture or position is liable to change vibration characteristics largely. On the other hand, if the holding area is increased, a part of vibrating region is pressed and both mechanical quality coefficients and coupling coefficients decrease to deteriorate vibration characteristics.
Because a piezoelectric vibrator is held directly with holders, a contact area between the vibrator and the holder and a holding position of the holders vary for each piezoelectric vibrator, and it is also a problem that the characteristic of the piezoelectric vibrator scatters. To sum up, it is difficult to hold a piezoelectric vibrator stably and reliably without affecting vibrator characteristics.
In order to solve the above-mentioned problems, it is proposed to form protrusions at positions for supporting or fixing a piezoelectric vibrator and to support it at the protrusions. This technique has an advantage that if protrusions can be formed correctly at a node, the position for supporting or fixing can be set surely around the node. Priorly, such protrusions are formed with a hard soldering process, a soldering process or a plating for a metallic material and with a printing process for an organic electrically conductive paint. The protrusions are made of an electrically conducting material because electrical leads are connected at the protrusions.
Japanese Utility Model laid open Publication 20422/1993 proposes a holding structure of piezoelectric vibrator wherein protrusions made of electrically conductive rectangular rubber are formed around a node point of vibrations of a piezoelectric vibrator and the piezoelectric vibrator is held at the protrusions. However, this holding structure has a problem that protrusions are deformed with a pressure applied for holding. Then, a contact area between the vibrator and the holders are not constant among piezoelectric vibrators, and the characteristics of the piezoelectric vibrator scatter broadly.
In recent years, the size of electronic apparatuses have been much reduced, so that a portable electronic apparatus such as a notebook-type personal computer are now widespread. Therefore, there is strong demand for the development of a high-performance acceleration sensor of a small size which may ensure and improve the reliability of electronic apparatuses of this type against impact given to such apparatuses. For example, an impact given to an apparatus which is going to write some data into a high-density hard disk would cause the positional deviation of the writing head thereof. Such positional deviation might result in a write error of the data or might damage the head. In order to prevent such accidents, it is necessary to detect the impact given to the hard disk thereby suspending the writing operation or allowing the head to escape to a safe position.
Conventionally, an acceleration sensor has generally been made of a piezoelectric material such as piezoelectric ceramic. An acceleration sensor of this type may accomplish a high detection sensitivity by utilizing the electromechanical transduction characteristics of a piezoelectric material used for the sensor. A piezoelectric acceleration sensor transduces the force given by the acceleration or the vibration into a voltage based on the piezoelectric effects so as to output the voltage. An acceleration sensor of this type is known by a rectangular bimorph piezoelectric element under a cantilever type structure disclosed in Japanese Laid-Open Patent Publication No. 2-248086. Such a cantilever type structure is shown in FIG. 36. As shown in FIG. 36, one end of the piezoelectric vibrator 311 with a bimorph structure is clamped by a holding member 313 with a conductive adhesive 315 or the like. Since the resonance frequency of such a bimorph piezoelectric element under the cantilever type structure is low, the element is used for measuring acceleration having relatively low frequency components. On the other hand, in the case of measuring the acceleration in a high frequency region, a rectangular bimorph piezoelectric element with a both ends clamped structure shown in FIG. 37 is used. As shown in FIG. 37, both ends of the piezoelectric vibrator 311 under a bimorph structure are clamped by the holding members 313 with an adhesive 315 or the like. The resonance frequency of the piezoelectric vibrator may be made relatively high by clamping both ends of the piezoelectric vibrator.
Although the acceleration sensor under the cantilever type structure mentioned above exhibits high sensitivity to the vibration in a low frequency region, the acceleration sensor exhibits low sensitivity to the vibration in a high frequency region and low impact-resistance. On the other hand, according to the acceleration sensor under the both ends clamped structure, a wide range of frequency region may be detected and the impact-resistance thereof is high, whereas, the sensitivity to the frequency is disadvantageously low and the downsizing thereof is very difficult.
In addition, in order to stabilize the sensitivity of a rectangular bimorph piezoelectric element, it is necessary to stabilize the resonance frequency thereof. Accordingly, the bimorph piezoelectric element is required to be held under an even stabler state. However, in actual use, some stress generated by mechanical or thermal variations causes a deviation in a portion supported by a metallic supporting member or in a portion clamped by a metallic holding member. For example, in the case of clamping the bimorph piezoelectric element by using an adhesive, the clamp point of the element changes depending on the application range of an adhesive. In addition, in accordance with the variation of the temperature of the adhesive, the clamping state of the element is varied, so that it becomes difficult to hold the element under a satisfactorily stable state.
An exemplary method for stably clamping a piezoelectric element is disclosed in Japanese Laid-Open Patent Publication No. 59-70923. In this patent publication, a slit is provided in a central portion of a bimorph piezoelectric vibrator, thereby forming a cantilever type structure so as to clamp the peripheral portion of the bimorph piezoelectric vibrator. According to this method, however, the area of the clamping portion, other than the cantilever type structure formed by the slit for contributing to the detection of the acceleration, is large, so that the size of the element also becomes disadvantageously large as a whole. Therefore, it is difficult to produce a small-sized acceleration sensor by this method.
In order to improve the sensitivity to acceleration, it is preferable to support the piezoelectric vibrator so that the inclination of the displacement of the vibrator, which is caused by the vibration, is limited at the supported point, as compared with completely clamping the piezoelectric vibrator so that the inclination of the displacement becomes zero at the clamp point. This is because the same acceleration gives a larger distortion to the piezoelectric vibrator supported by the former method, as compared with the distortion given to the piezoelectric vibrator completely clamped by the latter method. As a result, the output sensitivity of the piezoelectric vibrator held by the former method becomes larger. According to the conventional method mentioned above, however, it is difficult to stabilize the supporting state; the supporting state and the clamping state are interchangeable depending upon the fabrication conditions; the variation of the resonance frequency of the piezoelectric vibrator becomes large, and therefore the variation of the sensitivity to the acceleration also becomes adversely large.
Japanese Laid-Open Utility Model Publication No. 5-23617 discloses a holding structure for constituting a piezoelectric resonator in which a piezoelectric vibrator is held at square pillar shaped metallic protrusions formed on the surface of the piezoelectric vibrator. Unlike an acceleration sensor, the piezoelectric resonator disclosed in Japanese Laid-Open Utility Model Publication No. 5-23617 is an element utilizing the resonance phenomena of a vibrator, and therefore the metallic protrusions are provided at the position of the node of the vibrator so as not to obstruct the resonance of the vibrator. According to this utility model publication, the square pillar shaped metallic protrusions are formed by brazing, welding, melting, adhesion, or the like.
In the case of forming the metallic protrusions by brazing, welding or melting, the piezoelectric vibrator is heated at a high temperature during the formation of the protrusions, so that the characteristics of the piezoelectric vibrator deteriorate. In particular, in the case where piezoelectric ceramic is used as a material for the vibrator, the characteristics of the vibrator deteriorate greatly when it is heated to a high temperature, because the Curie temperature of a piezoelectric ceramic material is low and so the heat resistance thereof is low. The Curie temperature of a piezoelectric ceramic material used for a piezoelectric vibrator is approximately in the range of 250 to 350.degree. C. If a piezoelectric vibrator is heated at a temperature half as high as the Curie temperature (centigrade) or higher, i.e. in the range of 125 to 175.degree. C. or higher, then the characteristics of the piezoelectric vibrator deteriorate considerably. Therefore, in the case where the protrusions are formed on the piezoelectric vibrator by such a process as brazing and soldering in which the vibrator is heated at a high temperature, considerable deterioration of the piezoelectric vibrator is inevitable. FIG. 38 is a graph showing the relationship between the piezoelectric constant of a piezoelectric ceramic material and the heating temperature. In this case, the piezoelectric ceramic material with a Curie temperature of 300.degree. C. is heated for an hour. As shown in FIG. 38, the piezoelectric constant does not change until the heating temperature reaches 140.degree. C., but the piezoelectric constant decreases when the heating temperature is over 140.degree. C. As the heating temperature becomes higher, the piezoelectric constant decreases in shorter time (not shown in FIG. 38). In fact, if the protrusions are formed on the piezoelectric vibrator by a process such as brazing and soldering, then the characteristics of the piezoelectric vibrator are considerably degraded.
On the other hand, in the case where the metallic protrusions are formed on the piezoelectric vibrator by using an adhesive, the adhesive protrudes from the adhesion interface between the piezoelectric vibrator and the protrusions. Since the amount of the protruding adhesive is not constant, the variation and the deterioration of the characteristics of the vibrator are disadvantageously large.
In principle, it is possible to form the metallic protrusions by a plating process without heating the vibrator at a high temperature. However, it takes a long time to form a thick protrusion by a plating process, so that a protrusion with a thickness of only several .mu.m can be formed practically. If a piezoelectric vibrator made of a piezoelectric ceramic material is held at protrusions with such a small height, even only a slight inclination of the piezoelectric vibrator will make the vibrator come in contact with the holding member or the like for holding the protrusions, because the surface of a piezoelectric ceramic material has a ruggedness of 3 to 5 .mu.m. The contact of the piezoelectric vibrator with another member is equivalent to the displacement of the portion holding the piezoelectric vibrator, so that the length from the hold point of the piezoelectric vibrator to the extremity thereof changes, thereby varying the resonance frequency and the frequency characteristics of the piezoelectric vibrator. In the case of forming the protrusions by a printing process or the like using an organic material such as a conductive paste, it is also difficult to form thick protrusions, so that the same kinds of problems occur as those of the plating process. In addition, the protrusions obtained by a printing process have weak mechanical strength and weak adhesive strength. Accordingly, it is difficult to hold the piezoelectric vibrator stably.